KIT D816V and JAK2 V617F Point Mutations Are Recurrently Found In Suspected Hypereosinophilic Syndrome

Abstract 4100

In some patients with suspected hypereosinophilic syndrome (HES), clonality of eosinophils may be proven by identification of an acquired chromosome or molecular abnormality leading to the diagnosis of chronic eosinophilic leukemia (CEL). The most common molecular aberrations are fusion genes with involvement of PDGFRA, e.g. FIP1L1-PDGFRA (FP), or PDGFRB, e.g. ETV6-PDGFRB. Molecular testing for FP by RT-PCR or FISH is nowadays performed early in the diagnostic work-up of suspected non-reactive eosinophilia. However, eosinophilia is also present at variable frequency in patients with systemic mastocytosis (SM) and other subtypes of myeloproliferative neoplasms (MPN). Recurrent molecular markers for those entities are KIT D816V (80-90% positivity in SM) and JAK2 V617F (60-70% positivity in MPN). We therefore sought to evaluate the relative frequency of FP (by RT-PCR), KIT D816V (by D-HPLC plus direct sequencing) and JAK2 V617F (by ARMS-PCR) in 300 samples from patients with suspected HES/CEL and to correlate molecular findings with clinical features. Molecular abnormalities were identified in 42 (14%) cases; 22 (7.3%) were positive for FP, 14 (4.6%) for KIT D816V and 6 (2.0%) for JAK2 V617F, respectively. Most baseline clinical characteristics, e.g. leukocytes, absolute and relative number of eosinophils, hemoglobin, platelets or splenomegaly, were not different between the three entities.

Significant differences were found regarding age, gender, serum tryptase levels and course of disease. FP positive patients were significantly younger (p<0.001) and exclusively male while a female prepoderance was observed for KIT and JAK2 mutated patients. Significantly elevated serum tryptase levels (normal value <11.4μ g/l) were found in all cases of FP and KIT D816V positive patients. However, serum tryptase levels >50μ g/l were almost exclusively seen in KIT D816V positive SM patients. Aggressive SM (ASM) was diagnosed in 6 of 14 (43%) KIT D816V positive patients due to characteristic bone marrow morphology and the presence of diverse C-findings (e.g. anemia <10g/dl, n=2, and/or thrombocytopenia <100×10⁹/μ l, n=7). Frequent additional clinical features included lymphadenopathy (n=9) and urticaria pigmentosa (n=6). Two ASM patients died within first year of diagnosis while 21 (95%) FP positive CEL patients are in complete molecular remission on imatinib after a median treatment time of 28 months (range 8–149). We conclude that the serum tryptase level is an important diagnostic and prognostic marker in eosinophilia. We suggest that FP negative HES patients should be screened for KIT D816V and JAK2 V617F point mutations, both of which are potentially targetable by small molecule inhibitors.